Complimenting the usual array of guns are genetic powers called 'plasmids', similar to System Shock's psychic powers. Plasmids are controlled with the left trigger and bumper with weapons on the right. You soon find the electricity bolt, fireball and telekinesis powers, but the scope of these bolt-on upgrades is gigantic.
Plasmids are great to weaken and trick enemies, so a bit of back and fourth between guns and powers is essential. The interesting part is that you gradually create your own combat style. The Enrage plasmid tricks splicers into killing each other. You can use scenery to your advantage too; setting oil slicks aflame does the job a treat.
The deepest and most definitive part of the formula is the simple but involving RPG element. There are so many plasmid combinations and choices that a second play-through is guaranteed. Character and item building options are accessed via vending machines. Rarely-found 'Power to the People' stations give a one-time weapon upgrade ranging from damage increase to a larger clip size. Inventory stations - a nice idea - can similarly be used to build extra items such as ammo using bits and bobs scavenged from your journey.
Hacking also plays a major role and even these skills can even be upgraded through plasmids. Rapture's security system is immense and involves an entire network of cameras, turrets and flying bots that can be yours if successfully hacked. Oddly this is done through a complete rip off of Pipe Mania in a quick mini-game. You can even hack vending machines for cheaper goodies, which saves your wallet from taking a financial beating.
As for your plasmids and modifying tonics (which affect things like your max health and engineering attributes), you'll get those from special Gather Gardens - and the only way to craft them is by using Adam as credits. There's only one way to get it, and that's through the Little Sisters. And where there's a Little Sister, there's a Big Daddy...
Left alone these gigantic, diving-suited guerrillas are the most harmless inhabitants of Rapture. Cross them though, and you'll come face to face with one of the angriest characters we've ever encountered. You won't survive long enough to shout about it either.
Big Daddies guard the twisted, syringe-wielding little girls that harvest the only source of Adam in the game (there are two or three of them in each district, marked on the pause screen). Their relationship is both unsettling and touching at the same time. Little Sisters draw the Adam out of corpses, Big Daddies provide the backup.
These twisted little girls will skip, hold hands and beckon their metal protectors through Rapture's blood-splattered plazas, and the Daddy will give everything he's got to hug, protect and defend his tiny companion from anyone foolish enough to get too close. But if you want to bolster you're plasmids, you're going to have to piss him off.
The rules of gaming state that a hulking, heavily armed foe will be slower than a milk float with a flat. Safe in this fact, you'll likely unload a revolver in his face from 30ft down the hallway, and then shit your pants when he rushes towards you in a single, blurry dash before you realise you're still alive.
When you do die though Bioshock's death system brings you instantly back to life in a resuscitation chambers, meaning you can go straight back and continue the fight - with your enemy's health at whatever point you left it. Admittedly, this takes away some of the difficulty and tension of taking on the giants as there seems to be no real penalty for dying.